Catalyst for clarifying exhaust gas and method for preparation thereof, and exhaust gas clarification catalyst device for vehicle

ABSTRACT

A purification catalyst for exhaust gas enhances the activities of the precious metals, preventing drop of activities at high temperature, and exhibiting a sufficient performance even during low temperature (below 400° C.) operation when starting a vehicle or during idling. The Pd oxide is supported on the Al oxide, and the Al oxide is LnAlO 3  (Ln: rare-earth element).

CROSS REFERENCE TO RELATED APPLICATION

This application is a National Stage entry of International ApplicationNo. PCT/JP2004/011990, filed Aug. 20, 2004, the entire specificationclaims and drawings of which are incorporated herewith by reference.

TECHNICAL FIELD

The present invention relates to a purification catalyst for exhaustgas, to a production method therefor, and to a purification catalystdevice for exhaust gas, and specifically relates to a productiontechnique for a purification catalyst for exhaust gas in which nitrogenoxide (NOx), carbon hydride (HC), and carbon monoxide (CO) contained inan exhaust gas emitted from an internal combustion engine of anautomobile or the like during low-temperature operation can besimultaneously and effectively reduced so that the exhaust gas ispurified.

BACKGROUND ART

For purifying exhaust gas containing, for example, CO, HC, NO, and NO₂,precious metals (Pt, Rh, Pd and Ir) exhibit high performance. Therefore,it is preferable to employ the above-mentioned precious metals in thepurification catalyst for exhaust gas. These precious metals aregenerally mixed with or supported by Al₂O₃ of high surface-to-weightratio together with additives such as La, Ce, and Nd. On the other hand,composite oxides (for example, a perovskite-like oxide), made bycombining various elements, have extremely varied properties. Therefore,it is preferable for a purification catalyst for exhaust gas to employthe above-mentioned composite oxides. Moreover, when the precious metalis supported by the composite oxides, the properties of precious metalare significantly changed. From this viewpoint, a preferable performancefor purifying exhaust gas can be obtained in the purification catalystfor exhaust gas in which a precious metal is supported by a compositeoxide.

Various catalysts mentioned above are now being developed, and forexample, a technique in which a coalescence rate of the precious metalcan be reduced by having a perovskite be a support, judging fromdeterioration of the precious metal with reduction of active sites bycoagulation of the precious metal, is proposed (see claims of JapaneseUnexamined Application Publication No. 5-86259). Moreover, anothertechnique in which reduction of PdO can be reduced by using a perovskitein which the A site is defective, judging from reducing PdO which is anactivated species in a NO reduction reaction, whereby the PdO changes toPd which is low-active Pd, when the precious metal is Pd, is proposed(see the claims of reactions disclosed in Japanese UnexaminedApplication Publication No. 2003-175337). Usually, precious metals areused on a support of Al₂O₃ or the like, either alone or in combination,but in severe conditions such as in an automobile, active sites decreasedue to coagulation, and the activity drops substantially. To solve thisproblem, it is proposed to use precious metals together with otherelements in a form of composite oxides. As for Pd, in particular,composite oxides of rare-earth metals and Pd have been disclosed (seethe claims of Japanese Unexamined Application Publication No.S61-209045, the claims of Japanese Unexamined Application PublicationNo. H1-43347, the claims of Japanese Unexamined Application PublicationNo. H4-27433, the claims of Japanese Unexamined Application PublicationNo. H4-341343, the claims of Japanese Unexamined Application PublicationNo. H7-88372, and the claims of Japanese Unexamined ApplicationPublication No. H10-277393).

Conventional purification catalysts for exhaust gas exhibit sufficientperformance for reducing CO, HC, and NOx (NO, NO₂, etc.) contained inexhaust gas, in a running of vehicle, particularly during running athigh temperatures (not less than 400° C.). However, the conventionalcatalysts cannot exhibit sufficient performance for reducing CO, HC, andNOx, in a vehicle at the starting or idling thereof at low temperatures(below 400° C.).

As mentioned above, the reason that sufficient performance for purifyingthe exhaust gas cannot be obtained in the running at low temperature isas follows. That is, in the conventional purification catalyst forexhaust gas, a precious metal, for example, Pt, Rh, or Pd, is supportedon Al₂O₃ having a high surface-to-weight ratio. Due to the highsurface-to-weight ratio of the Al₂O₃, the precious metal isadvantageously supported in a highly dispersed condition. However, Al₂O₃is a stable compound, and does not mutually affect a supported preciousmetal, whereby activity of the precious metal is not improved.Accordingly, sufficient performance during the running at lowtemperature may not be obtained.

Moreover, in the running of a vehicle, it is preferable for Pd to existin a condition of PdO which is highly reactive. However, even if Pdsupported on the Al₂O₃ initially exists in a condition of PdO, the Pd isreduced to be in a metal condition at high temperatures (not less than900° C.), and as Pd coagulates, active sites decrease, whereby theactivity is significantly reduced.

DISCLOSURE OF THE INVENTION

The invention was made in light of the above demands, and it is hence anobject thereof to provide a purification catalyst for exhaust gas, inwhich activity of the precious metal is improved, and the reduction ofactivity at high temperatures is prevented, whereby sufficientperformance even during a vehicle starting up or idling at lowtemperatures (below 400° C.) can be obtained, and a production methodtherefor, and a purification catalyst device for exhaust gas.

The present inventors have intensively researched purification catalystsfor exhaust gas, in which sufficient performance, even in a vehiclestarting up or idling at low temperatures (below 400° C.), can beexhibited. As a result, maintenance of high activity in low temperatureoperation after exposure to high temperature has been discovered in thepurification catalyst for exhausts gas obtained by supporting Pd oxideon a support of perovskite composite oxide expressed as LnAlO₃ (Ln isany rare-earth element, including La, Ce, Pr, Nd, Pm, Sm, etc.) obtainedby baking a precursor salt of carboxylic complex polymer.

The present invention (the first aspect of the invention) was made inlight of the above knowledge. That is, a purification catalyst forexhaust gas of the present invention is a catalyst in which Pd issupported on an Al oxide, and the oxide is LnAlO₃ (Ln: rare-earthelement).

Moreover, the present inventors have also learned that a LaAlO₃ amongLnAlO₃ compounds, is trigonal or rhombohedral, and a B site in theperovskite is Al in the LaAlO₃, whereby the dipole moment of the LaAlO₃is large, and an electric fluctuation of PdO bounded on the LaAlO₃ islarger than that of PdO which exists independently. Therefore, theoxidation state of Pd in a surface of the PdO supported is a state ofPd²⁺ over a large area. This state is a preferable state for purifyingexhaust gas, whereby high activity at low temperatures can be obtained.Additionally, the present inventors have confirmed that this catalystcan exhibit high activity at low temperatures even after exposing thecatalyst to operating conditions of about 1000° C.

That is, in the above-mentioned purification catalyst for exhaust gas(the first invention), it is preferable that the aluminum oxide betrigonal or rhombohedral (the second aspect of the invention).

The inventors have attempted to combine the Pd oxide with a compositeoxide containing Pd and at least one rare-earth element (for example,Ln₂PdO₄), and support this composite oxide on the LnAlO₃ (Ln: rare-earthelement), and discovered that a higher activity at low temperature isobtained. More specifically, the Pd composite oxide is a compound of Pdoxide which is unstable at high temperature, and a stable rare-earthelement oxide. Therefore, in the Pd composite oxide, the oxidation stateof Pd is stabilized, and the oxidation state of Pd is Pd²⁺ in a largearea, which is preferred for purification of exhaust gas. As a result, ahigh purification activity of exhaust gas is obtained. In addition,since the Pd composite oxide can maintain the state of oxide up to about1100° C., a high heat resistance is realized. Moreover, the Pd compositeoxide is a compound of rare-earth elements not high in the degree ofcrystallinity and Pd, and the produced Pd composite oxide is particlesof low degree of crystallinity, and hence the dispersion of Pd is high.Hence, active sites increase, and a high purification activity ofexhaust gas is obtained. In addition, the catalyst of the inventionhaving the composite oxide supported on LnAlO₃ contains rare-earthelements in both composite oxides, and the contact surfaces of twocomposite oxides partly form solid solutions by way of the rare-earthelement, and the mobility of Pd composite oxide is lowered, and mutualcoagulation of Pd composite oxide particles is suppressed, and a highdurability is obtained.

The present invention (the third aspect of the inventions) was made inlight of the above knowledge. That is, preferably, the third aspects ofthe invention relates to the purification catalyst for exhaust gas inthe first or second aspect of the invention, in which the Pd oxidecontains at least Ln₂PdO₄ (Ln: rare-earth element). As the Pd compositeoxide, aside from Ln₂PdO₄, also, Ln₂Pd₂O₅, Ln₄PdO₇, etc., may becontained.

In the manufacturing process of LnAlO₃, the inventors have attempted toproduce a carboxylic complex polymer by evaporating and solidifying anaqueous solution of nitrate of constituent elements containingcarboxylic acid, and discovered that LnAlO₃ is produced in a singlephase, and further that the surface of LnAlO₃ easily interacts with Pdoxide when Pd oxide is supported. As a result, a high activity at lowtemperature is obtained in the purification catalyst for exhaust gashaving Pd oxide supported on LnAlO₃.

The present invention (the fourth and fifth aspects of the inventions)was made in light of the above knowledge. That is, in theabove-mentioned purification catalysts for exhaust gas (the first andthird aspects of the invention), it is preferable that at least one kindof compound selected from a group of compounds (carboxylic acid having ahydroxyl group or a mercapto group and having a carbon number of 2 to20, dicarboxylic acid having a carbon number of 2 or 3, andmonocarboxylic acid having a carbon number of 1 to 20) be added to anaqueous nitrate solution including a component, whereby a purificationcatalyst for exhaust gas is obtained (the fourth aspect of theinvention). Moreover, in the purification catalysts for exhaust gas (thefourth aspect of the invention), it is preferable that the aqueousnitrate solution be evaporated completely to obtain a carboxylic acidcomplex polymer, and that the carboxylic acid complex polymer be heated,whereby a purification catalyst for exhaust gas is obtained (the fifthaspect of the invention).

As the carboxylic acid having a hydroxyl group or a mercapto group andhaving a carbon number of 2 to 20, oxycarboxylic acid and a compound inwhich an oxygen atom in the hydroxyl of the oxycarboxylic acid isreplaced with a sulfur atom are cited. The carbon number of thesecarboxylic acids is 2 to 20 in light of solubility in water, ispreferably 2 to 12, is more preferably 2 to 8, and is most preferably 2to 6. Moreover, the carbon number of the monocarboxylic acid is 1 to 20in light of solubility in water, is preferably 1 to 12, is morepreferably 1 to 8, and is most preferably 1 to 6.

Furthermore, as concrete examples of the carboxylic acids having ahydroxyl group or a mercapto group and having a carbon number of 2 to20, for example, glycolic acid, mercaptosuccinic acid, thioglycolicacid, lactic acid, β-hydroxy propionic acid, malic acid, tartaric acid,citric acid, isocitric acid, allo-citric acid, gluconic acid, glyoxylicacid, glyceric acid, mandelic acid, tropic acid, benzilic acid, andsalicylic acid are cited. As concrete examples of the monocarboxylicacids, for example, formic acid, acetic acid, propionic acid, butyricacid, isobutyric acid, valeric acid, isovaleric acid, hexanoic acid,heptanoic acid, 2-methyl hexanoic acid, octanoic acid, 2-ethyl hexanoicacid, nonanoic acid, decanoic acid, and lauric acid are cited. In theabove-mentioned acids, it is preferable to use acetic acid, oxalic acid,malonic acid, glycolic acid, lactic acid, malic acid, tartaric acid,glyoxylic acid, citric acid, gluconic acid, and it is more preferable touse oxalic acid, malonic acid, glycolic acid, lactic acid, malic acid,tartaric acid, glyoxylic acid, citric acid, or gluconic acid.

Next, a production method for a purification catalyst for exhaust gas ofthe present invention (the sixth aspect of the invention) is a methodfor preferably producing the above-mentioned catalysts (the first tofifth aspects of the invention). That is, the sixth aspect of theinvention is a method in which when the purification catalyst forexhaust gas in which Pd oxide is supported on an aluminum oxide, atleast one kind of compound selected from a group of compounds(carboxylic acid having a hydroxyl group or a mercapto group and havinga carbon number of 2 to 20, a dicarboxylic acid having a carbon numberof 2 or 3, and a monocarboxylic acid having a carbon number of 1 to 20)is added to an aqueous nitrate solution including a component, whereby apurification catalyst for exhaust gas is obtained.

In the above-mentioned production method for a purification catalyst forexhaust gas (the sixth aspect of the invention), it is preferable thatthe aqueous nitrate solution be evaporated completely to obtain acarboxylic acid complex polymer, and that the carboxylic acid complexpolymer be heated (the seventh aspect of the invention), and it is morepreferable that the heating temperature be not more than 1000° C. (theeighth aspect of the invention).

The above purification catalyst for exhaust gas and its manufacturingmethod are the summary of the invention, but the inventors have furtherresearched specific applications of the first to eighth aspects of theinvention, and found that the purification catalyst for exhaust gas ofthe invention is particularly suited to an internal combustion enginefor an automobile, and have thereby completed a ninth aspect of theinvention.

The ninth aspect of the invention is a purification catalyst for exhaustgas for purifying exhaust gas from an automobile having Pd oxidesupported on Al oxide, in which the Al oxide is LnAlO₃ (Ln: rare-earthelement).

The purification catalyst for exhaust gas of the present invention inwhich PdO is supported on LnAlO₃ has a function in which the reductionof PdO to Pd metal can be decreased. The shape of Ln (rare-earth metal)variously changes in oxide states. For example, when a catalyst made bysupporting Pd on La₂O₃ is exposed to high temperature conditions, La₂O₃migrates onto the Pd grain from the contact area between Pd and La₂O₃,whereby a shape of filling up La₂O₃ with Pd is formed, resulting inadditional migration of minute amounts of La₂O₃ onto the Pd surface(Zhang et al., J. Phys. Chem., Vol. 100, No. 2, pp. 744-755, 1996). Evenin the present system (LnAlO₃), Ln and Pd form a complex compound,whereby reduction of PdO to Pd metal can be decreased. Owing to thiseffect, a purification catalyst for exhaust gas of the present inventioncan maintain a high activity state while running at low temperatures(below 400° C.).

Moreover, in the LnAlO₃, for example LaAlO₃ is characterized in that thecrystal system is trigonal or rhombohedral and the B site of perovskiteis Al. The trigonal or rhombohedral is, as shown in FIG. 1, a crystalsystem in which an ideal cubic system of a unit lattice is changed inthe c-axis direction, and the angle between the a-axis and the b-axis is120°. That is, the trigonal is a crystal system in which an ideal cubicsystem of a perovskite structure is significantly strained. In thecrystal system, the electron state among constituent atoms is extremelyunstable. In the rhombohedral system, as shown in FIG. 2, the trigonalsystem is expressed by a different basic axis, and the structure itselfis the same as in the trigonal system. FIG. 3 is an XRD spectrum as datademonstrating the difference in crystal systems of LaAl₃ supporting Pdor Pd oxide. That is, comparing the structures of LaAlO₃ and NdAlO₃, andGdAlO₃ which is another perovskite supporting Pd or Pd oxide inconventional purification catalyst for exhaust gas, as can be seen fromthe diagram, crystal systems of LaAlO₃ and NdAlO₃ are trigonal orrhombohedral, while the crystal system of GdAlO₃ is neither trigonal norrhombohedral, but is orthorhombic.

On the other hand, in the LaAlO₃, NdAlO₃, a B site in the perovskite isAl, whereby the bond between Al and O has a high degree of probabilityof being a covalent bond. Therefore, some of the dipole moment isgenerated in a crystal of perovskite which has generally a high degreeof probability of being an ionic bond. As described above, theperovskite, that is LaAlO₃, NdAlO₃, are trigonal or rhombohedral, and aB site in the perovskite-like composite oxides is Al in the oxides,whereby dipole moment of the oxides is larger than that of thewell-known purification catalyst for exhaust gas, for example LaFeO₃.

Due to the dipole moment, an electric fluctuation of PdO bound on theLaAlO₃ or NdAlO₃ is larger than that in which PdO exists independently.Therefore, the oxidation state of Pd in a surface of the PdO supportedis a state of Pd²⁺ over a large area. There are two oxidation states ofPd in a surface of the PdO, which are a state of Pd²⁺ and a state of Pd⁰(metal state). That is, in the purification catalysts for exhaust gas ofthe present invention in which PdO is supported on the LaAlO₃ or NdAlO₃,the oxidation state of Pd in a surface of the PdO is the state of Pd²⁺,whereby the catalysts of the present invention have high activity.Moreover, the catalysts of the present invention can exhibit highactivity during the running at low temperatures (below 400° C.) evenafter exposing the catalyst to an operating condition of about 1000° C.

Furthermore, when the LaAlO₃ or NdAlO₃ is produced, an aqueous nitratesolution of a component containing carboxylic acid is evaporatedcompletely to obtain a carboxylic acid complex polymer, and the polymeris heated at a relatively low temperature of 800° C., whereby LaA-lO₃ orNdAlO₃ are generated as a single phase.

On the other hand, when the LaAlO₃ or NdAlO₃ is produced in other ways,for example, solid-phase reaction, LaAlO₃ or NdAlO₃ is not generated asa single phase even if the heating at a relatively high temperature of1700° C. is performed (see Rare Earth Science, Kagaku-Dojin PublishingCompany, Inc, Ginya Adachi, p. 564). That is, LaAlO₃ or the like of thesingle phase can be synthesized at the above-mentioned low temperatureby using carboxylic acid. Therefore, sufficient surface-to-weight ratiocan be obtained, and the catalyst can be used in a state in which thesurface of the crystal lattice is active. In the purification catalystfor exhaust gas made by supporting Pd on the LnAlO₃ by using the methodof the present invention, sufficient surface-to-weight ratio and stronginteraction between LnAlO₃ and Pd can be obtained, whereby high activityat low temperatures can be realized.

In the case of Pd composite oxide (for example, Ln₂PdO₄) containing Pdand at least one rare-earth element used as the Pd oxide as aconstituent element of purification catalyst for exhaust gas of theinvention, the effects realized by this composite oxide are explainedbelow.

The Pd composite oxide is a composite compound of an unstable Pd oxideand a very stable oxide of a rare-earth element. For example, in thecase of PdO, the PdO surface may have two chemical states, Pd⁰ and Pd²⁺.In the Pd composite oxide, however, as a result of stabilization ofoxidation state by rare-earth element, the chemical state of thecompound outer surface is mostly Pd²⁺. Between Pd⁰ and Pd²⁺, since Pd²⁺is higher in activity, a high purification activity of exhaust gas isobtained in the Pd composite oxide.

Meanwhile, the decomposition temperature of PdO is about 800° C., butthe Pd composite oxide is stably present in an oxide state at 1100° C.Therefore, the Pd composite oxide has a high heat resistance. That is,Pd of which the oxide is not stable at high temperature is compoundedwith rare-earth element or alkaline earth element which is stable in anoxide state, and the Pd—O bond in the bulk is fortified. The Pdcomposite oxide is a composite compound of rare-earth element oralkaline earth element not high in degree of crystallinity and Pd.Hence, the produced Pd composite oxide is low in degree ofcrystallinity, and high in dispersion of Pd. As a result, active sitesare increased, and a high purification performance for exhaust gas isobtained. Further, when a composite oxide of rare-earth element and Pdis supported on a composite oxide composed of LnAlO₃, since rare-earthelements are contained in both composite oxides, the contact surfaces oftwo composite oxides partly form solid solution by way of the rare-earthelements, and the mobility of Pd composite oxide is lowered, and mutualcoagulation of Pd composite oxide particles is suppressed, and a highdurability is obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing an example of crystal system(trigonal) of Al oxide composing a purification catalyst for exhaust gasof the invention.

FIG. 2 is a perspective view showing an example of crystal system(rhombohedral) of Al oxide composing a purification catalyst for exhaustgas of the invention.

FIG. 3 is an XRD spectrum showing difference in crystal system ofvarious Al oxides on which Pd oxides are supported.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the present invention will be concretely explained byexamples.

MANUFACTURING EXAMPLE 1

Production of Composite Oxides as Support

Predetermined amounts of lanthanum nitrate hexahydrate and aluminumnitrate nonahydrate were dissolved in ion-exchanged water, whereby amixed solution was obtained. Next, a predetermined amount of malic acidwas dissolved in ion-exchanged water, whereby an aqueous malic acidsolution was obtained. These two solutions were mixed, the obtainedmixed solution was set on a hot plate with a stirrer, and the mixedsolution was heated to 250° C. and agitated by a stirring bar, wherebyevaporation of water into vapor was performed, complete evaporation wasperformed, and the dried sample was crushed into a powder by mortar andpestle. The crushed sample was moved to an aluminum crucible, the samplewas heated to 350° C. at a rate of 2.5° C./min in a muffle kiln, and aheat treatment was performed at 350° C. for 3 hours. Owing to the heattreatment, a provisional heated substance in which malate andnitrate-nitrogen (nitrate salt and nitrate ion) were removed wasobtained. After crushing the provisional heated substance into powderand mixing for 15 minutes by a mortar and pestle, the obtained mixturewas set in the aluminum crucible again, the sample was heated to 800° C.at a rate of 5° C./min in the muffle kiln, and a heat treatment wasperformed at 800° C. for 10 hours. Owing to the heat treatment, aperovskite-like composite oxide of which the composition was LaAlO₃ wasobtained.

Support of Pd Composite Oxide

A metal salt mixed aqueous solution was prepared by dissolvingpredetermined amounts of palladium nitrate dehydrate and lanthanumnitrate hexahydrate in ion-exchanged water. An aqueous solution of malicacid was prepared by dissolving a predetermined amount of malic acid inion-exchanged water. These two aqueous solutions were mixed, and thismixture and a predetermined amount of LaAlO₃ powder were put in aneggplant-shaped flask, and while evacuating the flask by a rotaryevaporator, the mixture was evaporated and solidified in a hot bath at60° C. By heating up to 250° C. at a rate of 2.5° C./min in a mufflekiln, the temperature was further raised to 720° C. at a rate of 5°C./min, and 750° C. was held for 3 hours. As a result, a catalyst powderof Manufacturing Example 1 of La₂PdO₄/LaAlO₃ having La₂PdO₄ impregnatedand supported on LaAlO3 was obtained. The specific surface area and Pddispersion degree of catalyst powder in Manufacturing Example 1 areshown in Table 1.

TABLE 1 Specific surface area by BET Pd dispersion (m²/g) degree (%)After After Initial endurance Initial endurance ManufacturingLa₂PdO₄/LaAlO₃ 9 5 17.0 2.4 Example 1 Manufacturing Nd₂PdO₄/LaAlO₃ 9 518.3 2.2 Example 2 Manufacturing Gd₂PdO₄/LaAlO₃ 8 5 19.2 2.1 Example 3Manufacturing La₂PdO₄/NdAlO₃ 9 5 17.1 2.8 Example 4 ManufacturingPd/Al₂O₃ 80 40 6.3 0.51 Example 5 Manufacturing Tb₂PdO₄/LaAlO₃ 1 1 10.10.72 Example 6 Manufacturing La₂PdO₄/GdAlO₃ 12 4 17.0 2.2 Example 7Estimation of Activity

Next, initial activities and activities after endurance running wereestimated for the obtained catalyst powders. The estimation wasperformed by flowing model exhaust gas of a vehicle into catalysts underconditions in which A/F (air-fuel ratio) was substantially 14.6 and SV(stroke volume) was 5000 h⁻¹. Endurance running was performed for 20hours at an endurance running temperature of 900° C. by using modelexhaust gas in which A/F (air-fuel ratio) was substantially 14.6. Theseresults are shown in Tables 2 and 3. That is, the Table 2 shows atemperature at which CO, HC, and NO are reduced by 50% in a temperatureincrease test of catalysts before the endurance running. Moreover, theTable 3 shows a temperature at which CO, HC, and NO are reduced by 50%in a temperature increase test of catalysts after the endurance running.

TABLE 2 Temperature at which CO, HC or NO are reduced by 50% CO HC NOManufacturing La₂PdO₄/LaAlO₃ 227 249 199 Example 1 ManufacturingND₂PdO₄/LaAlO₃ 221 243 198 Example 2 Manufacturing Gd₂PdO₄/LaAlO₃ 236258 204 Example 3 Manufacturing La₂PdO₄/NdAlO₃ 221 241 197 Example 4Manufacturing Pd/AL₂O₃ 276 287 252 Example 5 ManufacturingTb₂PdO₄/LaAlO₃ 249 268 239 Example 6 Manufacturing La₂PdO₄/GdAlO₃ 236257 209 Example 7

TABLE 3 Temperature at which CO, HC, or NO are reduced by 50% CO HC NOManufacturing La₂PdO₄/LaAlO₃ 306 312 240 Example 1 ManufacturingNd₂PdO₄/LaAlO₃ 298 302 241 Example 2 Manufacturing Gd₂PdO₄/LaAlO₃ 300303 245 Example 3 Manufacturing La₂PdO₄/NdAlO₃ 307 320 259 Example 4Manufacturing Pd/Al₂O₃ 326 335 >400 Example 5 ManufacturingTb₂PdO₄/LaAlO₃ 328 331 280 Example 6 Manufacturing La₂PdO₄/GdAlO₃ 336344 >400 Example 7

MANUFACTURING EXAMPLE 2

In the same manner as in Manufacturing Example 1, Nd₂PdO₄/LaAlO₃ wasmanufactured, and various estimations for activity were performed. Theresults are shown in Tables 1 to 3.

MANUFACTURING EXAMPLE 3

In the same manner as in Manufacturing Example 1, Gd₂PdO₄/LaAlO₃ wasmanufactured, and various estimations for activity were performed. Theresult is shown in Tables 1 to 3.

MANUFACTURING EXAMPLE 4

In the same manner as in Manufacturing Example 1, La₂PdO₄/NdAlO₃ wasmanufactured, and various estimations for activity were performed. Theresult is shown in Tables 1 to 3.

MANUFACTURING EXAMPLE 5

In the same manner as in Manufacturing Example 1, Pd/Al₂O₃ wasmanufactured, and various estimations for activity were performed. Theresult is shown in Table 1 to Table 3.

MANUFACTURING EXAMPLE 6

Predetermined amounts of lanthanum oxide and aluminum oxide were mixedby mortar and pestle, the mixed sample was moved to an aluminumcrucible, the sample was heated for 10 hours at 1100° C. in a mufflekiln, and LaAlO₃ was obtained by solid-phase reaction. Using this,Tb₂PdO₄ was supported in the same manner as in Manufacturing Example 1,Tb₂PdO₄/LaAlO₃ was manufactured. Various estimations for activity wereperformed for this catalyst. The results are also shown in Tables 1 to3.

MANUFACTURING EXAMPLE 7

In the same manner as in Manufacturing Example 1, La₂PdO₄/GdAlO₃ wasmanufactured, and various estimations for activity were performed. Theresult is shown in Tables 1 to 3.

According to the Tables 2 and 3, the purification catalysts for exhaustgas of the Manufacturing Example 1 to 3 exhibit excellent temperaturesat which CO, HC, and NO are reduced by 50% at any time before and afterthe endurance running. The reason for this is that the purificationcatalysts for exhaust gas of the Manufacturing Examples 1 to 3 are madeby supporting Pd on the LaAlO₃ (Ln: rare-earth material) and thesecatalysts have a property of suppressing a reduction of PdO to Pd athigh temperatures, whereby the high activity can be maintained in therunning at low temperatures after a running at high catalysttemperatures. Also in the purification catalysts for exhaust gas inManufacturing Examples 1 to 4, the crystal system of Al oxides istrigonal or rhombohedral, and the B site of perovskite is Al, and hencethe electric instability is great. Hence, Pd oxide adjacent to LaAlO₃ orNdAlO₃ is greater in electric fluctuation than an independent Pd oxide.Further, in the purification catalysts for exhaust gas in ManufacturingExamples 1 to 4, when manufacturing LaAlO₃ or NdAlO₃, by a process ofonce obtaining carboxylic complex polymer by evaporating and solidifyingthe aqueous solution of nitrate of constituent element containingcarboxylic acid, LaAlO₃ or NdAlO₃ is produced in a single phase, andwhen supporting Pd oxide, the surface state is likely to interact withPd oxide. In the process of manufacturing the mixed aqueous solution,malic acid is used, but the same effects are obtained by using citricacid or oxalic acid.

By contrast, in the purification catalysts for exhaust gas inManufacturing Examples 5 to 7, sufficient performance cannot be obtainedin low temperature operation as compared with the purification catalystsfor exhaust gas in Manufacturing Examples 1 to 4, and the reason is asfollows. In Manufacturing Example 5, Al₂O₃ is a stable compound, and itdoes not interact with the supported precious metal Pd, and the Pditself is not enhanced in activity. In the purification catalyst forexhaust gas in Manufacturing Example 6, although the crystal system ofAl oxide is trigonal or rhombohedral, since carboxylic acid is not usedin the manufacturing process of catalyst, LaAlO₃ of single phase cannotbe synthesized. Hence, sufficient specific surface area is not obtained,and the crystal lattice surface cannot be used in an active state. Inthe purification catalyst for exhaust gas in Manufacturing Example 7,the crystal system of Al oxide is orthorhombic, and the existence ofelectrons among component atoms is not as unstable as in the trigonal orrhombohedral system.

The purification catalyst for exhaust gas of the invention can beapplied in an internal combustion engine of an automobile or the like inwhich it is required to purify and reduce simultaneously and effectivelynitrogen oxide (NOx), carbon hydride (HC), and carbon monoxide (CO)contained in an exhaust gas.

1. A method for production of a purification catalyst for exhaust gas,wherein the purification catalyst comprises a Pd oxide consisting ofLn₂PdO₄ supported by LnAlO₃, wherein Ln is a rare-earth element, themethod comprising: providing at least one compound selected from thegroup consisting of compounds of carboxylic acid having a hydroxyl groupor a mercapto group and having a carbon number of 2 to 20, dicarboxylicacid having a carbon number of 2 or 3, and monocarboxylic acid having acarbon number of 1 to 20; and adding said at least one compound to anaqueous nitrate solution including Ln and Pd and an aqueous nitratesolution including Ln and Al.
 2. The method for production of apurification catalyst for exhaust gas according to claim 1, the methodfurther comprising: evaporating the aqueous nitrate solution completelyto produce a carboxylic acid complex polymer; and heating saidcarboxylic acid complex polymer.
 3. The method for production of apurification catalyst for exhaust gas according to claim 2, wherein aheating temperature in said heating of the carboxylic acid complexpolymer step is not more than 1000° C.